Wave motion compensating apparatus for use with floating hoisting systems

ABSTRACT

The dead end of the drilling line of the hoisting system of a floating drilling rig makes multiple passes between a stationary sheave assembly and a movable sheave assembly and then runs to a dead line anchor to which it is tied. A piston mechanism including a cylinder, piston, and piston rod is located on the floating rig structure and is coupled to the movable sheave assembly for controlling the spacing between the stationary and movable sheave assemblies. Fluid control apparatus communicates with the piston mechanism for controlling the fluid pressure therein for minimizing variations in the drilling line tension as the floating drilling rig or other type of floating structure rises and falls with the sea waves. The piston rod is subject to a pure tension load at all times, thereby enabling the rod to be of any suitable length to accommodate any wave height encountered.

United States Patent Bates, Jr. et al.

[54] WAVE MOTION COMPENSATING APPARATUS FOR USE WITH FLOATING HOISTING SYSTEMS [72] Inventors: Howard J. Bates, Jr., Houston, Tex.; Ado

Vujasinovie, Fullerton, Calif.

[73] Assignee: Joe Stine Inc., Houston, Tex.

[22] Filed: Nov. 17, 1969 [21] Appl. No.: 877.096

[451 Apr. 4, 1972 Richardson... .....254/l72 Gregory ..254/189 Primary Examiner-Harvey C. Hornsby Attorney-Jack W. Hayden and Richard E. Bee

[57] ABSTRACT The dead end of the drilling line of the hoisting system of a floating drilling rig makes multiple passes between a stationary sheave assembly and a movable sheave assembly and then runs to a dead line anchor to which it is tied. A piston mechanism including a cylinder, piston, and piston rod is located on the floating rig structure and is coupled to the movable sheave assembly for controlling the spacing between the stationary and movable sheave assemblies. Fluid control [56] References Cited apparatus communicates with the piston mechanism for con- UNITED STATES PATENTS trolling the fluid pressure therein tror minirnizing variations in L the drilling line tension as the floating drilling rig or other type 223,464 1/1880 l of floating structure rises and falls with the sea waves. The 2,109,297 2/1933 M 'y 254/173 3 piston rod is subject to a pure tension load at all times, thereby 2,896,582 7/1959 Justlce 91/4 enabling the rod to be of any suitable length to accommodate 2,916,017 12/1959 Camb ..91/4 any wave height encountere 3,259,371 7/1966 Goepfert..... ..254/173 3,260,508 7/1966 Powell ..254/186 4 Claims, 7 Drawing Figures I6 35 25 o L 36 "HI 20 ill r |r i 1 '11 g; M r lililllln It 24 mill.

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ATTORNEYS PATENTED 4 SHEET H []F 4 INVEN'IORS. Jul (1). #0 401- (i ATTORNEYS HOWARD J. BATES, JR.

A00 VuJAsnvovlc QJ- lll- WAVE MOTION COMPENSATING APPARATUS FOR USE WITH FLOATING I-IOISTING SYSTEMS BACKGROUND OF THE INVENTION This invention relates to wave motion compensating apparatus for use with floating hoisting systems such as those used on floating drilling rigs.

In the drilling of offshore oil and gas wells, an attempt is usually made to commence the drilling operation when the sea is relatively calm. Since several days or weeks may be required to drill the well, it sometimes happens that the sea will become quite rough before the well is completed. The rising and falling of the floating drilling rig under rough sea conditions presents problems. For example, it would be desirable to leave the drill string in the well bore so that drilling could be quickly resumed after the sea has calmed down again. If this is done, however, the rising and falling of the drill pipe during the rough sea conditions would tend to swab the well bore and could lead to an undesired blowout of the well. This can be prevented by activating the blow out preventer included in the wellhead equipment located on the ocean floor. When activated, the blowout preventer firmly engages and seals off around the drill pipe and allows only a very limited upward or downward movement thereof. This, however, causes problems aboard the floating drilling rig. In particular, since the drill string is now anchored to the ocean floor, the rising and falling of the rig places a considerable strain on the rig hoisting system and could, in fact, break the hoisting line. In addition, the upper part of the drill pipe might jam itself into the rig structure and damage the rig structure or the drill pipe or both.

These undesirable consequences could be avoided by providing some form of wave motion compensating device on board the drilling rig for maintaining a positive tension on the upper end of the drill pipe as the rig rises and falls and, at the same time, preventing such tension from exceeding the limits of the hoisting system. The use of such a device would enable the drill string to be left in the well bore so that drilling could again be commenced in fairly short order after the sea again becomes calm.

Various wave motion compensating arrangements have been heretofore proposed for use aboard a floating drilling rig. Some propose the use of rather involved systems of auxiliary sheave blocks and lines for connecting the customary travelling block to the drilling rig ina manner such that the travelling block will not rise and fall with the wave movement. Other prior proposals make use of special hydraulic piston mechanisms connected between the travelling block and the load or between the crown block and the top of the rig derrick for providing the wave motion compensation.

In general, these prior proposals tend to complicate the layout of the drilling rig hoisting system. They frequently require a substantial modification of the conventional hoisting system and sometimes require a modification of the derrick structure itself. In addition, many of the previously proposed devices are limited to use where the wave movements are relatively small, such as on the order ofa few feet or less. They are not readily capable of accommodating wave movements on the order of or feet or more.

A further form of device which is pertinent to the present discussion is the hydropneumatic cable tensioner described in US. Pat. No. 3,314,657, granted to Messrs. Prudhomme and Reynolds on Apr. 18, 1967. This device employs a movable sheave block, a stationary sheave block and a piston mechanism located therebetween for purposes of controlling the tension in a cable or rope reeved between the two sheave blocks. While of general utility, this device has not heretofore been adapted for use in solving the particular problem presently being considered.

SUMMARY OF THE INVENTION It is an object of the invention, therefore, to provide new and improved wave motion compensating systems and apparatus for use with floating hoisting systems and which substantially avoid one or more of the foregoing limitations of the arrangements heretofore proposed.

It is another object of the invention to provide new and im proved hoisting system wave motion compensating apparatus for use with a floating drilling rig and which requires very little modification of the conventional drilling rig hoisting system.

It is a further object of the invention to provide new and improved hoisting system wave motion compensating apparatus for use with a floating drilling rig and which can be used when the drill string extending from the underwater well-bore is clamped by the water bottom blowout preventer.

In accordance with the invention, wave motion compensating apparatus for controlling the tension in the hoisting line of a hoisting system located on a floating structure comprises sheave structure for location on the floating structure adjacent the hoisting system. In accordance with one feature of the invention, this sheave structure includes a frame structure adapted to be mounted on the deck of the floating structure, a first sheave assembly mounted at one end of the frame structure, track means carried by the frame structure, carriage means adapted for movement on the track means and a second sheave assembly carried by the carriage means. The first and second sheave assemblies are adapted to receive the dead end of the hoisting line and to pass same back and forth therebetween before passing it to a dead line anchor. The apparatus also includes a piston mechanism coupled to the movable or carriage mounted sheave assembly for controlling the position thereof. The apparatus further includes fluid control means for passing pressurized fluid to and from the piston mechanism for controlling the tension in the hoisting line. In accordance with another feature of this invention, the fluid control means includes fluid reservoir means for passing pressurized fluid to and from the piston mechanism and valve means for controlling the movement of such fluid between the reservoir means and the piston mechanism.

For a better understanding of the present invention, together with other and further objects and features thereof, reference is had to the following description taken in connection with the accompanying drawings, the scope of the invention being pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS Referring to the drawings:

FIG. 1 shows a floating drilling rig including a simplified diagrammatic representation of wave motion compensating apparatus constructed in accordance with the present invention;

FIG. 2 is an elevational view showing in greater detail the major mechanical structures of the wave motion compensating apparatus shown diagrammatically in FIG. 1;

FIG. 3 is an elevational view looking from the right-hand end of the apparatus of FIG. 2;

FIG. 4 is a partial plan view of the FIG. 2 apparatus with some of the structure removed to show some of the underlying structure;

FIG. 5 is an enlarged partially cross-sectional view of a portion of the FIG. 2 apparatus;

FIG. 6 is a cross-sectional view of a portion of the FIG. 5 apparatus taken along section line 66 thereof; and

FIG. 7 is a schematic diagram showing in greater detail the pneumatic, hydraulic and electrical systems used with the wave motion compensating apparatus of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the diagrammatic illustration of FIG. 1, there is shown a drilling rig 10 located on a floating structure 11 floating at the surface of a body of water 12. The submerged land mass underlying the body of water 12 is indicated at 13. The drilling rig 10 includes a derrick structure 14 for supporting a hoisting system designated generally at 15. The hoisting system 15 includes drawworks 16, a drilling line 17, a crown block 18, a travelling block 19 and a dead line anchor 20. The

drilling line 17 passes from the winch drum of the drawworks 16, upwardly to the crown block 18 and then makes multiple loops back and forth between the crown block 18 and the travelling block 19. The drilling line 17 then runs downwardly from the crown block 18, through a sheave portion of wave motion compensating apparatus 21 and to the dead line anchor 20, to which it is tied or otherwise secured. In its downward journey from the crown block 18, the drilling line 17 passes over a weight indicator 22 which provides an indication ofthe tension in the drilling line 17.

Operation of the winch mechanism of drawworks 16 serves to reel in or pay out the drilling line 17, depending on the direction of operation thereof, thus causing a raising or lowering of the travelling block 19.

Hung from the hook of the travelling block 19 is a swivel mechanism 23 which is connected to the top end of a drilling kelly 24. Kelly 24 passes downwardly through a rotary table 25 and is connected to the top end of a string of drill pipe 26. Drill pipe 26 extends downwardly through the water 12, passes through a blowout preventer 27 and into a borehole 28 being drilled into the subbottom soil material 13. A drill bit 29 is attached to the bottom end of the drill pipe 26. The upper portion of the borehole 28 is lined with a protective pipe 30 and the blowout preventer 27 is mounted on the upper end of such pipe 30. It is noted that the showing of the wellhead equipment located at the bottom of the body of water 12 has, for sake of simplicity, been considerably simplified. Among other things, the control lines running from the floating structure 11 to the blowout preventer 27 have been omitted.

The wave motion compensating apparatus 21 includes sheave structure 31 located on the deck of the floating structure 11 to one side of the derrick 14. Major components of this sheave structure 31 include a stationary sheave assembly 32 and a movable sheave assembly 33, between which the hoisting line 17 makes multiple loops back and forth before it passes to the dead line anchor 20. The wave motion compensating apparatus 21 also includes a piston mechanism 34 coupled to the movable sheave assembly 33 for controlling the position thereof with respect to the stationary sheave assembly 32. This piston mechanism 34 includes a piston cylinder 35, a piston 36 and a piston rod 37 for coupling the piston 36 to the movable sheave assembly 33. The wave motion compensating apparatus 21 further includes fluid control apparatus 38 for passing pressurized fluid to and from the piston mechanism 34. Major components of this fluid control apparatus include a high pressure fluid reservoir 39 communicating with the rod side of the piston cylinder 35, a low pressure fluid reservoir 40 communicating with the opposite or nonrod side of the piston cylinder 35 and a control panel 41 for controlling and monitoring the operation of the fluid reservoirs 39 and 40.

Referring now to FIGS. 2-6, there will be considered in detail a preferred form of construction for the sheave structure 31. As indicated in FIG. 2, the sheave structure 31 includes a frame structure 42 which is bolted to the deck of the floating structure 11. This frame structure 42 includes an upper pair of longitudinal beams 43, a lower pair of longitudinal beams 44 and forward and rearward pairs of vertical connecting beams 45 and 46. The two resulting side frames are tied together by periodic tie plates, an upper one being indicated at 47 in FIG. 4 and a lower one being indicated at 48 in FIGS. 3 and 4. As indicated in FIG. 4, the lower plate 48 is adapted to be bolted to the deck of the floating structure 11. This construction provides a narrow elongated box-like form of frame structure.

As indicated in FIGS. 3 and 4, the fixed or stationary sheave assembly 32 includes a set of six side-by-side sheaves 50 rotatably mounted on a common shaft or axle 51. Sheaves 50 rotate independently of one another. The axle 51 is journaled in a pair of bearing blocks 52 which are secured to the vertical front end beams 45. If desired, the sheave assembly 32 can take the form of a conventional crown block type of sheave assembly.

Secured to and extending along the inner sides of the longitudinal beams 43 and 44 are upper and lower pairs of carriage tracks 53 and 54 which are, perhaps, best seen in FIG. 5. In FIG. 5, the nearer two of the beams 43 and 44 have been omitted. As indicated in the cross-sectional view of FIG. 6 for the lower rear carriage track 54, each of the carriage tracks is mounted on the upper side of the lower interior flange portion of its longitudinal beam. These carriage tracks 53 and 54 extend the length of the longitudinal beams 43 and 44.

As indicated in FIGS. 2 and 5, the sheave structure 31 further includes a wheeled carriage 55 which is adapted for movement on the carriage tracks 53 and 54. Carriage 55 includes a box-like center structure formed by vertical beams 56 and horizontal beams 57. A lower wheel truck 58 is secured to the underside of the box-like central structure, while an upper wheel truck 59 is secured to the upper side of such structure. Wheels 60 and 61, located at the four corners of the wheel trucks 58 and 59, respectively, ride on the carriage tracks 53 and 54, the lower wheels 60 riding on the lower tracks 54 and the upper wheels 61 riding on the upper tracks 53. Wheels 60 are mounted on axles 62, while wheels 61 are mounted on axles 63. These axles extend laterally across the respective wheel trucks 58 and 59 and are rotatably supported by such wheel trucks 58 and 59.

As best seen in FIG. 4, the movable sheave assembly 33 includes a set of five side-by-side sheaves 64 rotatably mounted on a common axle 65. Sheaves 64 rotate independently of one another. Axle 65 is journaled in a pair of bearing blocks 66 which are mounted on the rearward side of the movable carriage 55, the closer one of these bearing blocks 66 being visible in FIGS. 2 and 5. If desired, such sheave assembly 33 may take the form of a conventional crown block type of sheave assembly. I

As indicated in FIG. 2, the carriage 55 is coupled to the piston rod 37 by means ofa yoke 67 and a clevis 68, the legs of the yoke 67 being pinned to the central structure of the carriage 55 and the clevis 68 being mounted on the end of the piston rod 37 and pinned to the yoke 67. As best seen in FIG. 5, a bumper plate 69 is mounted on the forward end of the central structure of the carriage 55. This bumper plate 69 limits the forward movement of the carriage 55 by butting against the rearward sides of the vertical front end beams 45 of the frame structure 42. A cooperating stationary bumper plate may be mounted on and extend across between the rearward sides of such vertical front end beams 45.

Considering now the piston mechanism 34, such mechanism 34 is located adjacent the rearward end of the frame structure 42, as indicated in FIG. 2. The longitudinal center axis of the elongated piston cylinder 35 is in approximate alignment with the longitudinal center axis of the frame structure 42. The forward end of the cylinder 35 is secured to the rearward end of the frame structure 42 and the rearward end of the cylinder 35 is supported by a support member 70. As further indicated in FIG. 2, the high pressure fluid reservoir 39 is in the form of an elongated cylindrical tank and is mounted on top of the frame structure 42. The low pressure fluid reservoir 40, on the other hand, is in the form of a cylindrical tank mounted on top of the piston mechanism 34.

Referring now to FIG. 7, there is shown in greater detail the pneumatic, hydraulic and electrical systems associated with the wave motion compensating apparatus 21. It is initially noted that both sides of the piston cylinder 35 and the lower portions of the fluid reservoirs 39 and 40 are filled with oil. The oil in reservoir 39 is indicated at 71, while the oil in reservoir 40 is indicated at 72. The remainder of the high pressure reservoir 39 is filled with high pressure air. This high pressure air is obtained from an air compressor 73 which is connected to the reservoir 39 by way of a compressor air check valve 74, an air shutoff valve 75 and an air conduit 76. A vent pipe 77 is connected to the conduit 76 by way of a vent pipe shutoff valve 78. The free end of vent pipe 77 is open to the atmosphere. The air compressor 73 includes an electrical motor which is energized by electricity supplied thereto by an electrical supply line 79, a start-stop switch 80 and an electrical line 81. The electrical supply line 79 is connected to the normal electrical system of the drilling rig. Associated with the air compressor 73 is a pressure switch 82 which senses the pressure on the output line of the compressor 73 and turns the compressor 73 on and off, as necessary, to keep this pressure constant. Compressor 73 is capable of providing pressurized air at a pressure of, for example, 2,500 pounds per square inch.

Coupled to the high pressure reservoir 39 is a pneumatic relief valve 83 for releasing air from the reservoir 39 when the pressure in such reservoir 39 exceeds a predetermined limit established by the release mechanism in the valve 83. This release limit may be set at a value of, for example, 2,000 pounds per square inch Relief valve 83 sets the maximum desired system pressure. The high pressure reservoir 39 is also provided with a normally closed drain valve 84.

The high pressure reservoir 39 is fluidly coupled to the rod side of the piston cylinder 35 by means of a remote controlled valve mechanism 85. This valve mechanism 85 includes a pair of pilot operated check valves 86 and 87 connected in parallel with one another. The flow lines above the check valves 86 and 87 are designated by the reference numeral 88, while those below the check valves 86 and 87 are designated by the reference numeral 89. The pilot mechanisms in the check valves 86 and 87 are pneumatically controlled by high pressure air which is selectively supplied thereto by way of an air line 90, an on-off control valve 91 and an air line 92, the latter being connected to the high pressure air line 76. When the control valve 91 is closed, no air pressure actuating signal is supplied to the pilot mechanisms in the check valves 86 and 87. In the absence of such an actuating signal, the check valves 86 and 87 function as check valves to only allow the passage offluid from the piston cylinder 35 to the reservoir 39 and not vice versa. When the control valve 91 is set to its open position, the air pressure actuating signal is supplied to the valves 86 and 87 and functions to keep the check mechanisms in such valves 86 and 87 in a continuously open condition. This enables fluid to flow in either direction through the valves 86 and 87. As will be more fully appreciated hereinafter, valve mechanism 85 provides means for quickly activating the wave motion compensating apparatus 21.

A pressure gauge 93 is located in the control panel 41 and is connected to the high pressure line 92 by way ofa gauge shutoff valve 94. Pressure gauge 93 monitors the fluid pressure in the reservoir 39.

Considering now the low pressure reservoir 40, the remainder of such reservoir 40 is filled with low pressure air. This low pressure air is obtained from the drilling rig air supply system and is supplied to the reservoir 40 by way of an air supply line 95, an air filter 96, an air pressure regulator 97 and an air line 98. A pressure gauge 99 is connected to the air line 98 by way ofa gauge shutoff valve 100. Gauge 99 monitors the fluid pressure in the low pressure reservoir 40. The air pressure provided by the rig air supply may be on the order of, for example, 250 pounds per square inch. A pneumatic relief valve 101 is coupled to the low pressure reservoir 40 for releasing air from such reservoir 40 when the pressure therein exceeds a predetermined limit set by the release mechanism in the relief valve 101. Such release limit may be set at, for example, 200 pounds per square inch. Reservoir 40 is also provided with a normally closed drain valve 102.

The low pressure reservoir 40 is fluidly coupled to the opposite or nonrod side of the piston cylinder 35 by means of a fluid conduit 103, a valve mechanism 104 and a fluid conduit 105. Valve mechanism 104 includes a flow control valve 106 and a check valve 107 connected in parallel therewith. Check valve 107 enables fluid flow therethrough from the reservoir 40 to the piston cylinder 35 but does not allow fluid flow in the reverse direction. Valve 106 is set or sized to restrict the rate of flow of the oil from the piston cylinder 35 to the reservoir 40 when the piston rod 37 is being retracted. This limits the rate of retraction of such piston rod 37 and serves to prevent damage to the piston mechanism 34 in the event of a sudden decrease in the tension in the drilling line 17. Check valve 107 together with the flow valve 106, on the other hand, cooperate to enable a much greater rate of flow when the piston rod 37 is being extended.

The wave motion compensating apparatus 21 further includes an electricalindicator system for remotely monitoring the position of the movable sheave assembly 33 within the frame structure 42. This indicator or position monitoring system includes a position indicator display 108 located on the control panel 41 and comprised of a series of electrical lamps 109, each of which denotes a different position of the carriage 55 within the frame structure 42. The position monitoring system further includes carriage position sensing means represented by a series of spaced apart microswitches 110 which are mounted on the frame structure 42. These microswitches 110 are connected by different sets of conductors in an electrical cable 111 to different ones of the indicator lamps 109 on the control panel 41. The microswitches 110 are tripped, one at a time, by a trip bar 112 (FIGS. 3 and 5) which is mounted on and extends from the side of the carriage 55. When one of the microswitches 110 is tripped, it causes a particular one of the electrical lamps 109 to be lighted, the remainder of the lamps 109 remaining unlit. The length of the trip bar 112, together with the spacing of the microswitches 110, is preferably selected so that one of the lamps 109 will always be lit .to reveal the present position of the carriage 55.

The weight indicator 22 which measures the tension in the drilling line 17 includes a roller 113 rotatably mounted on pivoted arms 114 and 115 for engaging the drilling line 17. A strain gauge transducer 116 is mounted on the arm 115 for sensing the stress in such arm 115 and producing an electrical signal proportional thereto. The stress in arm 115 increases as the tension in cable 17 increases. The electrical signal generated by the transducer 116 is carried by an electrical line 117 to an electrical gauge or meter 118 which may, for convenience, be located on the control panel 41. Meter 118 provides a visual indication proportional to the tension in drilling line 17.

OPERATION OF THE PREFERRED EMBODIMENT A principal use of the wave motion compensating apparatus 21 described above is as a safety device to prevent damage to the structure of the drilling rig 10 or to the drill string 26 when the blowout preventer 27 is activated to clamp the drill pipe 26 and, thus, to anchor it to the ocean floor 13. In such case, it need not be used when the sea is relatively calm and the normal drilling of the borehole 28 is being performed. Assuming this to be the desired manner of usage, the wave motion compensating apparatus 21 may be deactivated during the normal drilling operation by setting the air compressor shutoff valve 75 to its closed to and opening the vent pipe shutoff valve 78. This releases the pressure from the high pressure reservoir 39. This causes the piston 36 to move to the right so as to completely extend the piston rod 37. In this condition, the movable sheave carriage 55 abuts against the vertical front end beams 45 (FIG. 2) at the right-hand end of the frame structure 42. In this position, the compensating apparatus 21 has no effect on the performance of the drilling operation.

Once the piston rod 37 is completely extended for purposes of deactivating the compensating apparatus 21, it is then desired to preset or cock the compensating apparatus 21 so that it can be quickly reactivated if conditions should warrant. This is accomplished by closing the pilot control valve 91 and the vent pipe shutoff valve 78. After this is done, the compressor shutoff valve 75 is reopened to again pressurize the interior of the high pressure reservoir 39. It is assumed, of course, that the air compressor 73 is turned on and is operative for supplying the high pressure air. At this time, the valves 86 and 87 function as check valves to prevent the oil 71 in the reservoir 39 from flowing back into the piston cylinder 35.

Assume now that sea conditions become so rough that it becomes desirable to discontinue the drilling operation and to activate the blowout preventer 27 to clamp the drill pipe 26. Before the blowout preventer 27 is activated, the wave motion compensating apparatus 21 is activated to bring it into operation. This is quickly accomplished by simply opening the pilot control valve 91. This supplies the actuating pressure to the pilot mechanism in the valves 86 and 87 for purposes of opening the valves 86 and 87 so as to enable fluid flow in both directions therethrough. This enables the pressurized oil in the high pressure reservoir 39 to flow into the rod side of the piston cylinder 35 and to move the piston 36 toward the left. This moves the movable sheave carriage 55 away from the stationary sheave assembly 32. When the carriage 55 reaches approximately its mid-position and the drilling rig floating structure 11 is about midway between a wave peak and a wave trough, the blowout preventer 27 is activated to clamp the drill pipe 26. Thereafter, the compensating apparatus 21 performs its wave motion compensating function.

Considering now such wave compensating function, when the drilling rig floating structure 1 1 falls during the trough of a sea wave, the tension in the drilling line 17 tends to decrease. The pressure in the high pressure reservoir 39 thereupon forces more oil into the piston cylinder 35, thus forcing the piston 36 and hence the movable sheave carriage 55 toward the left. This movement takes up the slack in the drilling line 17 in a manner which offsets the decrease in tension which would otherwise be caused by the falling of the floating structure 11. As a consequence, the tension in the drilling line 17 remains very nearly constant.

When, on the other hand, the drilling rig floating structure 11 rises with the crest ofa sea wave, the tension in the drilling line 17 tends to increase. This causes a movement of the movable sheave carriage 55 back toward the right. This, in turn, moves the piston 36 to the right and forces oil back into the high pressure reservoir 39. If this movement should cause the pressure in the reservoir 39 to exceed the limit set back by the relief valve 83, then such relief valve 83 releases sufficient air from the reservoir 39 to maintain the pressure therein at this limit. The movement of the movable sheave carriage 55 toward the right decreases the tension in the drilling line 17 in a manner which offsets the increase caused by the upward movement of the floating structure 11, the net change in tension being approximately zero.

As thus seen, the wave motion compensating apparatus 21 serves to keep the tension in the drilling line 17 substantially constant. This means that a positive tension will always be maintained on the travelling block 19 and the drill string 26 connected thereto. It further means that the hoisting system 15 will not be subjected to an excessive stress when the floating structure 11 rises with the crest ofa sea wave.

In a rough or approximate sense, the wave motion compensating apparatus 21 is like a large spring connected between the end of the drilling line 17 and the dead line anchor 20. Such spring" serves to expand and contract with the rise and fall of the sea waves to keep the drilling line 17 taut without breaking it.

After the sea has died down sufficiently to allow drilling to be resumed, the wave motion compensating apparatus 21 can again be disabled by closing the compressor shutoff valve 75 and opening the vent pipe shutoff valve 78.

From the foregoing, it is seen that the rod end of the piston cylinder 35 is the working chamber of such cylinder 35. The high pressure fluid in such chamber is the principal load bearing and tension controlling medium.

The presence of the oil on both sides of the piston cylinder 35 provides continuous lubrication for the inner wall of the piston cylinder 35. The presence of the oil on the nonrod side of the piston 36, together with the use of the low pressure reservoir 40, provides a dampening of the movement of the piston 36 to prevent any sustained high frequency back and forth oscillations of the piston 36 following an abrupt change in drilling line tension. The use of the direction sensitive valve mechanism 104 intermediate the piston cylinder 35 and the low pressure reservoir 40 to limit the rate of movement of oil from the cylinder 35 to the reservoir 40 serves to prevent damage to the piston mechanism 34 in the event of a sudden decrease in the drilling line tension. The use of the pilotoperated check valves 86 and 87 also provides a further safety feature. In particular, if the pilot control valve 91 should be shut off without the piston rod 37 being completely extended, the check valves 86 and 87 nevertheless allow oil to move from the cylinder 35 to the reservoir 39 in the event of the occurrence of an increase in drilling line tension, which occurrence produces an extending force on the piston rod 37 if it is not completely extended.

The maximum peak-to-trough amplitude of sea wave movement which can be accommodated by the wave motion compensating apparatus 21 is determined by the length of the piston cylinder 35 (or the length of the frame structure 42, if less) and the number of loops made by the drilling line 17 between the stationary sheave assembly 32 and the movable sheave assembly 33. If, for example, the drilling line 17 makes the same number of loops between the sheave assemblies 32 and 33 as it does between the crown block 18 and the travelling block 19, then there is a l-to-l ratio between the movement of the travelling block 19 and the movement of the movable sheave assembly 33. in this case, a piston cylinder having an effective operating length or, more accurately, a rod stroke of 15 feet would enable the wave compensating apparatus 21 to provide compensation for sea waves as large as 15 feet in height.

The present invention is constructed so that the high pressure is exerted on the piston rod 37 to apply and maintain a pure tension force on the rod. Since there is no compression loading of the rod 37, the problem of buckling of the rod is eliminated and therefore its length can be increased to accommodate the highest sea waves.

While there has been described what is at present considered to be a preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, intended to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. Wave motion compensating apparatus for controlling the tension in the hoisting line of a hoisting system in response to upward and downward movement of a floating structure on which the hoisting system is located comprising:

a. a frame structure for location on the deck of the floating structure adjacent the hoisting system;

b. a first sheave assembly mounted at one end of said frame structure;

0. track means carried by said frame structure;

d. carriage means adapted for movement on said track means;

e. a second sheave assembly carried by said carriage means;

said first and second sheave assemblies being adapted to receive the dead end of the hoisting line and to pass same back and forth therebetween before passing it to a dead line anchor;

g. a piston mechanism coupled to said carriage means for controlling the position thereof on said track means, said piston mechanism including:

1. a piston cylinder for location on the deck of the floating structure adjacent said frame structure;

2. a piston movable within said cylinder;

3. and a piston rod for coupling said piston to said carriage means;

h. and fluid control means for passing pressurized fluid to and from the piston mechanism for controlling the tension in the hoisting line in response to upward and downward movement of the floating structure with the waves, said fluid control means including:

1. high pressure fluid reservoir means communicating with the rod side of said piston cylinder;

2. fluid supply means for supplying high pressure fluid to said high pressure reservoir means;

3. low pressure fluid reservoir means communicating with the piston side of said piston cylinder;

4. fluid supply means for supplying low pressure fluid to the piston side of said piston cylinder;

5. relief valve means communicating with said high pressure reservoir means for releasing fluid therefrom when the pressure therein exceeds a predetermined limit;

6. remote controlled valve means for controlling the movement of fluid between said high pressure reservoir means and said piston cylinder on the rod side thereof;

7. and means for selectively supplying an actuating signal to said remote controlled valve means, said remote controlled valve means being comprised of pilot operated check valve means and the actuating signal is in the form of a fluid pressure signal, the check valve means, when not actuated, functioning as a check valve and, when actuated, functioning to allow passage of fluid in both directions between said high pressure reservoir means and said piston cylinder on the rod side thereof.

2. Apparatus in accordance with claim 1 wherein both sides of the piston cylinder and the lower portions of both of the reservoir means are filled with oil and the two fluid supply means supply pressurized air for filling the remainder of their respective reservoir means.

3. Apparatus in accordance with claim 1 wherein said fluid control means further includes valve means for regulating the movement of fluid between the low pressure fluid reservoir means and said piston cylinder, such valve means including means for providing a greater restriction on the flow of fluid in one direction than in the other.

4. Apparatus in accordance with claim 1 wherein said fluid control means further includes valve means for regulating the movement of fluid between the low pressure fluid reservoir means and the piston cylinder, such valve means including means for passing fluid in both directions and check valve means connected in parallel therewith for providing a second flow path only for fluid flowing from the low pressure fluid reservoir means to said piston cylinder.

. Dated April 4, 1972 HOWARD J. BATES. JR. et a1 error appears in the above-Menti Letters Patent are hereby correctec'i as shown below:

fied patent Column 6, line 55, remove "to" after "closed" and insert position--.

Signed and sealed this 18th day of July 1 72.

(SEAL) Attest EDWARD M.FLETGHER,J' R. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

1. Wave motion compensating apparatus for controlling the tension in the hoisting line of a hoisting system in response to upward and downward movement of a floating structure on which the hoisting system is located comprising: a. a frame structure for location on the deck of the floating structure adjacent the hoisting system; b. a first sheave assembly mounted at one end of said frame structure; c. track means carried by said frame structure; d. carriage means adapted for movement on said track means; e. a second sheave assembly carried by said carriage means; f. said first and second sheave assemblies being adapted to receive the dead end of the hoisting line and to pass same back and forth therebetween before passing it to a dead line anchor; g. a piston mechanism coupled to said carriage means for controlling the position thereof on said track means, said piston mechanism including:
 1. a piston cylinder for location on the deck of the floating structure adjacent said frame structure;
 2. a piston movable within said cylinder;
 3. and a piston rod for coupling said piston to said carriage means; h. and fluid control means for passing pressurized fluid to and from the piston mechanism for controlling the tension in the hoisting line in response to upward and downward movement of the floating structure with the waves, said fluid control means including:
 1. high pressure fluid reservoir means communicating with the rod side of said piston cylinder;
 2. fluid supply means for supplying high pressure fluid to said high pressure reservoir means;
 3. low pressure fluid reservoir means communicating with the piston side of said piston cylinder;
 4. fluid supply means for supplying low pressure fluid to the piston side of said piston cylinder;
 5. relief valve means communicating with said high pressure reservoir means for releasing fluid therefrom when the pressure therein exceeds a predetermined limit;
 6. remote controlled valve means for controlling the movement of fluid between said high pressure reservoir means and said piston cylinder on the rod side thereof;
 7. and means for selectively supplying an actuating signal to said remote controlled valve means, said remote controlled valve means being comprised of pilot operated check valve means and the actuating signal is in the form of a fluid pressure signal, the check valve means, when not actuated, functioning as a check valve and, when actuated, functioning to allow passage of fluid in both directions between said high pressure reservoir means and said piston cylinder on the rod side thereof.
 2. a piston movable within said cylinder;
 2. Apparatus in accordance with claim 1 wherein both sides of the piston cylinder and the lower portions of both of the reservoir means are filled with oil and the two fluid supply means supply pressurized air for filling the remainder of their respective reservoir means.
 2. fluid supply means for supplying high pressure fluid to said high pressure reservoir means;
 3. low pressure fluid reservoir means communicating with the piston side of said piston cylinder;
 3. Apparatus in accordance with claim 1 wherein said fluid control means further includes valve means for regulating the movement of fluid between the low pressure fluid reservoir means and said piston cylinder, such valve means including means for providing a greater restriction on the flow of fluid in one direction than in the other.
 3. and a piston rod for coupling said piston to said carriage means; h. and fluid control means for passing pressurized fluid to and from the piston mechanism for controlling the tension in the hoisting line in response to upward and downward movement of the floating structure with the waves, said fluid control means including:
 4. Apparatus in accordance with claim 1 wherein said fluid control means further includes valve means for regulating the movement of fluid between the low pressure fluid reservoir means and the piston cylinder, such valve means including means for passing fluid in both directions and check valve means connected in parallel therewith for providing a second flow path only for fluid flowing from the low pressure fluid reservoir means to said piston cylinder.
 4. fluid supply means for supplying low pressure fluid to the piston side of said piston cylinder;
 5. relief valve means communicating with said high pressure reservoir means for releasing fluid therefrom when the pressure therein exceeds a predetermined limit;
 6. remote controlled valve means for controlling the movement of fluid between said high pressure reservoir means and said piston cylinder on the rod side thereof;
 7. and means for selectively supplying an actuating signal to said remote controlled valve means, said remote controlled valve means being comprised of pilot operated check valve means and the actuating signal is in the form of a fluid pressure signal, the check valve means, when not actuated, functioning as a check valve and, when actuated, functioning to allow passage of fluid in both directions between said high pressure reservoir means and said piston cylinder on the rod side thereof. 